This invention relates in general to hearing assistive devices, and more particularly to hearing assistive devices that actively identify and reduce electronic interference induced in the hearing assistive device circuitry.
Generally speaking, hearing assistive devices including hearing aids worn outside the ear and cochlear implants operate in either a microphone mode, in which sound waves incident upon the device are converted to electrical energy, or a telecoil mode, in which magnetic energy is converted to electrical energy. In either mode, the resultant electrical signal is subsequently amplified, processed, and output to the user. When a hearing aid operates in an environment that includes a modulated RF field or a fluctuating magnetic field, undesired interference may be induced in the hearing aid circuitry as the varying fields are detected and processed as electrical signals by the hearing aid. Because digital devices such as digital cell phones produce RF emissions, the rapidly expanding use of digital wireless communication devices has made electronic interference at hearing assistive devices an increasingly significant problem.
Digital wireless telephones transmit over a wireless network via radio waves. The radio waves generated by the digital telephone are typically detected and demodulated by the hearing aid circuitry, thereby introducing an interference signal to the hearing assistive device. The interference signal is then amplified, processed, and delivered to the user along with the desired signal. As a result, the audible quality of the desired signal is diminished. Digital wireless devices that employ time division multiplexed modulation schemes often generate interference due to the on/off keying of their modulation envelopes. The pulsing of the transmissions may produce interference at the fundamental frequencies associated with the pulse rates, as well as at the associated harmonic frequencies across the audible spectrum. Interference may also be produced by RF energy picked up by components of hearing assistive devices, such as a telecoil in a hearing aid.
The digital telephone's electronics, such as the backlighting, the display, the keypad, the battery leads and the circuit board often also generate pulsed magnetic fields. The resultant magnetic field energy is typically combined with, for example, a hearing aid's wiring and interconnections, to generate interference at the hearing aid. This type of interference, often referred to as baseband magnetic interference, is also converted to an electrical signal that is then processed by the hearing aid, amplified, and delivered to the hearing aid user along with the desired signal, such as the voice of a human speaker. In addition to digital cell phones, digital cordless phones, portable digital radios and other digital devices generate electromagnetic interference which, when processed by the hearing aid, is subsequently output to the user. Analog apparatus such as power transformers, fluorescent lighting, and power lines likewise produce electromagnetic field static that interferes with hearing assistive devices.
Electronic interference, whether generated by pulsating electric or magnetic fields, combines with the desired signals picked up by a microphone, telecoil, or circuitry to form a composite signal at the hearing assistive device. The composite signal is processed by the hearing assistive device and output to the user. Depending on the source and duration of the interference, the hearing assistive device performance may be noticeably and significantly reduced, to the point where the hearing impaired user is discouraged from either using the hearing assistive device, such as a hearing aid, or discouraged from using the item that generates the interference, such as a cellular telephone.